While multicolour printing on paper and comparable substrates can today be essentially regarded as quite feasible, the person skilled in the art is still faced with great difficulties in the case of the multicolour printing of textile substrates if the printing involves difficult designs or colour tones or if the printed colours are required to have a high degree of brilliancy.
In general, the screen printing process is used for printing on textile substrates. In that process, each printing ink involved in the printing process requires its own screen printing stencil. Screen printing stencils may be completely open or, in order to obtain half-tones, they may also be in grid-form analogously to grid printing plates in offset printing; the term used in the latter case is grid screen printing stencils. The grid percentage of a grid screen printing stencil is, again analogously to offset printing, the ratio of the open area to the entire surface area of a notional elemental area of the grid screen printing stencil.
When printing on paper and comparable substrates, the entire design to be printed is made up of a limited set of, as a rule, four standard printing inks (generally cyan, yellow, magenta and black). The case of special colours is of no importance with regard to the following and has accordingly not been taken into account. The printing original carrying the design to be printed is scanned photoelectrically picture element by picture element by a lithographic scanning device, and so-called colour separations are established (nowadays electronically by digital image-processing) from the measurement data obtained, which separations represent the amounts of the four standard printing inks in the design. Those colour separations are then used to produce grid films which serve to produce the individual printing plates for the four printing inks involved.
That procedure, which has proved successful in offset printing (and, in a slightly modified form, also in intaglio and flexo printing), can be used at best to only a limited extent on textile substrates owing to the completely different colour build-up and in most cases gives highly unsatisfactory printing results. Accordingly, other methods are normally used for the multicolour printing of textile substrates.
The printing original or the design to be printed carried thereon is split into individual areas with each of which is associated an individual printing ink that is as close as possible to the actual colour tone. A screen printing stencil that is open in all of the areas of the design that belong to the printing ink in question is then produced for each of the printing inks provided. Nowadays, the production of the screen printing stencils is likewise effected using photoelectric image-scanning devices (lithographic scanners) supported by digital image-processing with devices analogous to those used in the production of offset printing plates.
For reasons of cost and for reasons of practicability, there is in practice a limit to the number of printing inks that can be used in this process. On the one hand the production of screen printing stencils is time-consuming and accordingly expensive per se and, on the other, customary screen printing arrangements are not generally set up for more than approximately ten printing inks. In practice, therefore, the process is subject to limitations which often lead to unsatisfactory printing results, especially if a relatively high reproduction quality is required. In addition, special effects, such as, for example, finely graded colour tone variations or shades and 3D effects, cannot be achieved or are difficult to achieve with this known process. Furthermore, critical colour tones, such as, for example, skin colours, cannot be produced with this process without streaking.